1. Field of the Invention
The present invention relates to a coextruded laminate structure which is superior in gas barrier properties and interlaminar strength, and more particularly it relates to a laminate structure formed by laminating (A) a layer of saponified product of ethylene-vinyl acetate copolymer and (B) a layer of hydrophobic thermoplastic resin, especially saturated polyester resin, one over the other, with (C) a layer of ethylene-acrylate ester polymer of specific composition interposed therebetween.
2. Description of the Prior Art
With the recent remarkable technical innovation in the food packaging industry, thermoplastic resins such as saturated polyester have come into the market because of their hygienic qualities, good appearance, and lightness which leads to a reduction of transportation cost. Saturated polyesters, however, are not suitable for long storage of foods and carbonated beverages because of their poor gas barrier properties. For this reason, glass bottles and metal cans are predominant in this area, but they are not completely satisfactory because they cause environmental pollution when thrown away on the road and require a great deal of expense for recovery after use. These factors promoted the development of a new packaging material.
The saponified product of ethylene-vinyl acetate copolymer containing 20 to 55 mol% of ethylene is superior in transparency and gas barrier properties which are desirable for food packaging; however, it is not necessarily satisfactory because of its insufficient stiffness and moisture resistance. On the other hand, saturated polyester resin, particularly hydrophobic thermoplastic resin composed mainly of polyethylene terephthalate has a good balance between impact resistance and stiffness and is superior in hygienic qualities. Nevertheless, it finds only limited use in food packaging because of its insufficient gas barrier properties (for oxygen and carbon dioxide).
In order to obtain a food packaging material having both of the superior gas barrier properties of the saponified product of ethylene-vinyl acetate copolymer and the superior mechanical properties of saturated polyester resin, it is conceivable that layers of both resins may be laminated one over the other. However, it is impossible to make a laminate by simple heat bonding because the two polymers have no affinity for each other.
It is disclosed in Japanese Patent Laid-open No. 71556/1980 that there is adhesion between a polymer composed of ethylene, acrylate ester, and ethylenic carboxylic acid or anhydride thereof and a saturated polyester resin. According to this disclosure, the former polymer is produced by chemically bonding an ethylenic unsaturated fatty acid or anhydride thereof to a copolymer of ethylene and carboxyl-containing monomer, such as ethylene-vinyl acetate copolymer, ethylene-acrylate ester copolymer, and ethylene-methacrylate ester copolymer. There is demonstrated in the specification that such a copolymer can be bonded to a saturated polyester resin.
It is disclosed in Japanese Patent Laid-open No. 101883/1979 that a thermoplastic resin can be laminated to the saponified product of ethylene-vinyl acetate copolymer with an interlayer of modified copolymer placed therebetween. The modified copolymer is obtained by chemically bonding an ethylenic unsaturated fatty acid or anhydride thereof to a copolymer of ethylene and carboxyl-containing or substituted carboxyl-containing monomer.
There is disclosed in Japanese Patent Laid-open No. 110282/1979 a laminate structure having a layer of saponified product of ethylene-vinyl acetate copolymer. In this case, the bonding interlayer is a metal-modified copolymer which is produced by chemically bonding an ethylenic unsaturated carboxylic acid or anhydride thereof to a copolymer of ethylene and carboxyl-containing or substituted carboxyl-containing monomer, followed by reaction with a metal salt.
As mentioned above, the bonding interlayer for lamination is made of a copolymer composed of ethylene, acrylate ester, and ethylenic carboxylic acid or anhydride thereof. Such a bonding layer is selective in adhesion depending on the type of layers to be bonded. This makes it necessary to select a proper bonding interlayer according to individual layers.
If a polymer with high adhesion composed of ethylene, acrylate ester, and ethylenic unsaturated carboxylic acid or anhydride thereof is used alone for bonding layers of saponified product of ethylene-vinyl acetate copolymer and saturated polyester resin, the resulting laminate is poor in appearance due to irregularities and streaks. If the copolymer is replaced by the one having low adhesion to improve the appearance of the laminate, it is not suitable for coextrusion to be run at a high speed, although it provides sufficient adhesion at a low lamination rate.